This invention relates broadly to a method and an apparatus for detecting faults on any one of a plurality of adjacent conductors utilising magnetic field measurements. The invention will be described herein with reference to fault indicators for overland power distribution cables, it will be appreciated, however, that the invention does have broader applications for various conductor arrangements in which faults may be experienced.
Equipment for detection and location of faults on a plurality of overland power distribution cables involves typically the measurement of the magnetic field produced by the alternating currents flowing in the cables, using a single magnetic field sensing coil. Such equipment derives a signal indicative of the average magnetic field, and this is used for the detection and location of faults. When one or more of the cables experience a fault, high currents flow in the cables, producing a rapid increase in the magnetic fiend around the cables. Therefore, if an increase is detected by the equipment, this is indicative of a fault current in one or more of the cables having passed the magnetic field sensing coil.
However, the applicant has found that the use of a single coil has the disadvantage that there is usually at least one set of fault currents for a given cable configuration that will produce a change in the magnetic field around the cables that is not detectable by the single coil. This is because a single coil measurement cannot be utilised to monitor changes in the magnetic field that vary the relevant magnetic field vector at the point of the measurements in a direction perpendicular to the central axis of the coil.
In accordance with a first aspect of the present invention there is provided a method of detecting for a fault on one of a plurality of adjacent, alternating current carrying conductors; the method comprising the steps of detecting the waveform of a first component of a magnetic field vector representative of the magnetic field around the plurality of conductors at a location along the conductors, detecting the waveform of a second ( i.e., angularly displaced) component of the magnetic field vector; monitoring, at the location, the two waveforms over a continuous time interval for any changes in the waveforms; and detecting for a fault on one of the plurality of the conductors as indicated by a change over the continuous time interval in either one or both of the waveforms.
Preferably, the method further comprises the step of determining whether the fault experienced is a phase to phase or a phase to earth fault as indicated by a change in both waveforms.
In one embodiment, the first and second components are at 90xc2x0 with respect to each other.
Advantageously, the first component is the horizontal component of the magnetic field vector and the second component is the vertical component of the magnetic field vector.
In one embodiment, the step of monitoring the two waveforms comprises the steps of monitoring a first amplitude of the first waveform over a period of the alternating current, and monitoring a second amplitude of the second waveform over the period, and the step of detecting for a fault comprises detecting for the fault as indicated by a change in either one or both of the amplitudes.
Preferably, the step of monitoring the two waveforms comprises the step of monitoring a phase difference between the first and second waveforms, and the step of detecting for a fault comprises detecting for the fault as indicated by a change in the phase difference.
In accordance with a second aspect of the present invention there is provided an apparatus for detecting for a fault on one of a plurality of adjacent, alternating current carrying conductors, the apparatus comprising first detecting means for detecting the waveform of a first component of a magnetic field vector representing the magnetic field around the plurality of conductors at a location along the conductors; second detecting means for detecting the waveform of a second (i.e., angularly displaced) component of the magnetic field vector; monitoring means for monitoring the two waveforms over a continuous time interval for any changes in the waveforms; and a detecting unit arranged to generate a fault indication signal depending on a change over the continuous time interval in either one or both of the waveforms.
Preferably, the detecting unit is further arranged to determine whether the fault experienced is a phase to phase or phase to earth fault depending on reference data stored in a database of the apparatus.
In one embodiment, the monitoring unit is arranged to monitor a first amplitude of the first waveform over a period of the alternating current, and to monitor a second amplitude of the second waveform over the period, and the detecting unit generates a fault indication signal depending on a change in either one or both of the amplitudes.
Preferably, the monitor unit is further arranged to monitor a phase difference between the first and second waveforms and the detecting unit is further arranged to generate the fault indication signal depending on changes in the phase difference.
The present invention may be more readily understood from the description of preferred forms of an apparatus for electrical measurements on conductors given below with reference to the accompanying drawings.